refactor(observability): drop file-SD writer for http_sd_configs

Slice 4 of observability-service-registry. Removes the shared-file
Prometheus bridge; external Prometheus now consumes node-exporter targets
via http_sd_configs against GET /api/monitoring/prometheus-targets.

- services/targets.py: removed write_prometheus_targets() (the file
  writer) and its json/Path/get_settings imports; updated module docstring.
  build_node_exporter_targets() is unchanged and still powers the HTTP
  endpoint.
- main.py: removed the startup write_prometheus_targets call.
- routers/settings.py: removed the _write_prometheus_targets helper and
  its three post machine create/update/delete call sites + the now-unused
  targets import.
- config.py: removed the prometheus_file_sd_dir field.
- docker-compose.yml / docker-compose.dev.yml: removed the
  PROMETHEUS_FILE_SD_DIR backend env var.
- tests: removed TestWritePrometheusTargets + the write_prometheus_targets
  import in test_targets.py; rewrote the two TestSettingsMachines tests to
  assert machines appear/disappear from /api/monitoring/prometheus-targets
  (the surviving HTTP path) instead of the removed file-writer side effect.

ruff clean; 239 backend tests pass.

frontend/src/components/ObservabilityPage.tsx

@@ -621,33 +621,33 @@ export function ObservabilityPage() {
 						</Select>
 					</CardHeader>
 					<CardContent className="space-y-4">
-					{selectedMachine ? (
-						GRAFANA_BASE_URL ? (
-							<>
-								<GrafanaLinkCard
-									title={`${selectedMachine.name} metrics`}
-									description="Open the Node Exporter overview dashboard for this machine in Grafana."
-									href={nodeExporterDashboardUrl}
+						{selectedMachine ? (
+							GRAFANA_BASE_URL ? (
+								<>
+									<GrafanaLinkCard
+										title={`${selectedMachine.name} metrics`}
+										description="Open the Node Exporter overview dashboard for this machine in Grafana."
+										href={nodeExporterDashboardUrl}
+									/>
+									<GrafanaLinkCard
+										title={`${selectedMachine.name} logs`}
+										description="Explore Loki logs for this machine in Grafana."
+										href={logsUrl}
+									/>
+								</>
+							) : (
+								<EmptyState
+									icon={Gauge}
+									title="No Grafana service configured"
+									description="Add a Grafana service instance to enable deep-links to dashboards and logs."
+									action={
+										<Button variant="outline" size="sm" asChild>
+											<Link to="/services">Open Services</Link>
+										</Button>
+									}
 								/>
-								<GrafanaLinkCard
-									title={`${selectedMachine.name} logs`}
-									description="Explore Loki logs for this machine in Grafana."
-									href={logsUrl}
-								/>
-							</>
+							)
 						) : (
-							<EmptyState
-								icon={Gauge}
-								title="No Grafana service configured"
-								description="Add a Grafana service instance to enable deep-links to dashboards and logs."
-								action={
-									<Button variant="outline" size="sm" asChild>
-										<Link to="/services">Open Services</Link>
-									</Button>
-								}
-							/>
-						)
-					) : (
 							<EmptyState
 								icon={ServerOff}
 								title="No machine selected"